1. Field of the Invention
The present invention relates generally to a wireless communication system, and more particularly, to a wireless communication system for transmitting and receiving data through both licensed and unlicensed bands.
2. Description of the Related Art Spectrum bandwidth is a precious resource in wireless communication. Licensed bandwidth includes partitions of bandwidth that are licensed to operators to provide specific wireless services. Unlicensed bandwidth includes other partitions of bandwidth that are not specifically assigned to any operator, and accordingly, any entity may use these unassigned partitions as long as predefined requirements are met. Each operator usually provides authorized services exclusively through licensed bands (i.e., authorized services are typically provided through spectrum that may not be used by any other operators), and thus inter-operator interference may be completely avoided. However, since unlicensed bands are open and every operator or personal can access the bandwidth, interference control is very important with respect to unlicensed bands. For example, Wireless Fidelity (WiFi) technologies are deployed over unlicensed band based on a collision-detection and contention mechanism. When the number of WiFi users increases, a significant portion of resources may be wasted on contention and collision. Further, due to the contention nature of WiFi, Quality of Service (QoS) of the data service may not be guaranteed.
As stated above, licensed bands generally provide services with less interference and better QoS than services provided over unlicensed bands, which low-cost services with fair performance in exchange for less reliable or robust channel conditions due to contention and interference.
In packet-switched data networks, between layers (such as between an application layer and a top-most layer), the layers pass Service Data Units (SDU) across the interfaces. The application layer (or another higher layer) understands the structure of the data in the SDU, but the lower layer at the interface does not understand the structure. Instead, the lower layer at the interface treats the SDU as payload, operating to transport the SDU to the same interface at the destination. In order to transport the SDU, a protocol layer will add, to the SDU, certain data the protocol layer needs to perform its function. For example, the protocol layer might add a port number to identify an application, a network address to help with routing, a code to identify the type of data in the packet of the SDU and error-checking information. All this additional information, as well as the original SDU from the higher layer, constitutes a Protocol Data Unit (PDU) at this layer. When the PDU passes over an interface from a layer that constructed the PDU to a layer that merely delivers the PDU, the PDU becomes a service data unit to that layer. The process of adding addressing and control information (which is also called encapsulation) to an SDU to form a PDU and the passing of that PDU to the next lower layer as an SDU is repeated until a lowest layer is reached and the data passes over some medium as a physical signal.